1. Field of the Invention
The present invention relates to a circuit arrangement for load regulation of circuit components arranged in a receive path of a transponder, a detector circuit for signal detection, in particular for passive and/or semi-passive transponders, a transponder, and a method for operating the transponder. The invention resides in the field of transponder technology and more particularly in the field of contactless communication for the purposes of identification.
2. Description of the Background Art
In passive and semi-passive transponders, an electromagnetic signal sent out by a base station is received and demodulated by the transponder. Passive transponders have no energy supply of their own, so the energy required in the transponder for demodulation and decoding of the received electromagnetic signal must be extracted from this electromagnetic signal itself. Energy can be extracted from an electromagnetic wave either inductively by an antenna coil, or magnetically using a dipole antenna. For general background on this technology, known as RFID technology, please refer to the “RFID Handbuch” by Klaus Finkenzeller, third revised edition, 2002.
In passive 125 KHz systems currently in use, the extraction of energy in the electromagnetic near field is implemented through inductive coupling. The energetic range achieved in this way lies in the range of a few centimeters to approximately 0.5 m. In order to achieve a greater range and a higher data transmission rate for the data transmission, carrier frequencies in the UHF frequency band or microwaves are increasingly used in the area of RFID technology. Dipole antennas are typically used for coupling the energy and the data signal at such high carrier signal frequencies. Ranges of up to a few meters can be achieved with passive transponders using such dipole antennas.
A goal in present and future RFID systems is to achieve the greatest possible ranges at the highest possible data transmission rates with passive transponders. A long range can be achieved, in particular, by increasing the transmit power of the base station. Since RFID systems generate and radiate electromagnetic waves, and thus can be considered radio installations, an important boundary condition of, for example, national and European HF regulations is that these RFID systems must not interfere with or impair other radio services. The required consideration of other radio services severely limits the selection of operating frequency for RFID systems as well as their transmit power. On the basis of these national and European HF regulations, the maximum transmit power is sharply limited with respect to the frequency in question.
FIG. 1 shows a schematic block diagram of a known detector circuit 1 that is arranged in a receive path of a transponder. The detector circuit 1 contains an input-side dipole antenna 2 for receiving a transmitted high-frequency carrier signal XHF. As a function of the field strength of the carrier signal XHF, a high frequency signal V1 is generated by the dipole antenna 2 and is fed to a rectifier 3 which follows the dipole antenna 2. A signal capacitor 4 is arranged between the outputs of the rectifier 3. A voltage U1 that is derived from the received and rectified signal V1, and which thus is a measure of the field strength of the high frequency carrier signal XHF, drops across the signal capacitor 4. This signal voltage U1 firstly contains the data which is present as modulation of the high frequency carrier XHF. In addition, the signal voltage U1 also contains the energy for the transponder's energy supply. An analysis circuit 5 for analyzing the signal U1, and thus extracting the data, is also provided.
In most RFID systems, data transmission takes place with the use of pulse-interval modulated signals. In such systems, digital data are exchanged between the base station and the transponder by the amplitude-modulated carrier wave XHF. The individual data bits are produced through pulse-interval modulation of the carrier signal XHF in that the transmitter in the base station switches an electromagnetic field on for specific time intervals and then off again. When the transponder receives the carrier signal modulated in this way, a signal voltage U1 derived from the field strength of the carrier signal XHF is generated on the input side of the transponder; this signal voltage has voltage dips at the points where the electromagnetic field was switched off on the transmitter side. Such a voltage dip is also referred to hereinafter as a “notch.” The data now lie in the time interval between two such voltage dips. The length of such a time interval thus determines the value of the corresponding data bit. For example, provision can be made here that a first time interval corresponds to a logic “0” and a second time interval that is longer than the first time interval corresponds to a logic “1”. The field gap in which the base station transmitter is switched off, and therefore transmits no electromagnetic carrier signal, thus in a certain sense represents a separator between two successive data bits.
Increasingly stringent security requirements in identification necessitate ever higher data transmission rates in modern RFID systems in order to keep the time periods during which an identification takes place as short as possible, in order to thereby transmit a large number of information packets modulated on a carrier wave in ever shorter periods of time. Consequently, ever increasing ranges for data communication are required in RFID systems operating at low power levels, regardless of the limited transmit power. In order to satisfy this requirement, the transponder must extract adequate energy from the field of the transmitted carrier signal XHF even with very weak electrical and/or magnetic fields. However, this is only possible when the rectifier 3 of the transponder has the highest possible efficiency. Moreover, it must also be possible to detect and reproduce even very small signal voltages U1.
Modern transponders must therefore be capable of operating both in a near field where a large electric field of the carrier signal is present, and in a far field where the electric field is sometimes very strongly attenuated. However, this operation of a transponder in both the near field and far field gives rise to the following problem, which is described on the basis of FIGS. 2a and 2b: 
FIGS. 2a and 2b show the behavior of the envelope curve of the transmitted high frequency carrier signal XHF and the behavior of the signal voltage U1, which must reproduce this envelope curve as well as possible. FIG. 2(a) shows the curve behavior in a transponder operated in the far field, and FIG. 2(b) shows the curve behavior in a transponder operated in the near field. In each case, a represents the notch of the envelope curve and of the signal voltage; b designates the envelope curve of the high frequency carrier signal XHF, and c designates the signal voltage U1 reproducing this carrier signal. The solid line labeled c represents the characteristic curve of the signal voltage in the case of a low discharge current, and the dashed line labeled c represents the corresponding signal voltage characteristic at a high discharge current. The following considerations apply: the signal voltage c should reproduce the behavior of the carrier signal and its envelope curve b as well as possible, with it being important here that the voltage dips a in the signal voltage c functioning as separators are also very pronounced.
The envelope curve b of the carrier signal XHF has, on the one hand, a low amplitude for a transponder operated in the far field, and on the other hand forms an adequately wide notch a (FIG. 2(a)). In order to still be able to detect the corresponding voltage dips a with such low voltage amplitudes of the signal voltage U1 here, the detector 1 of the transponder typically has a short time constant so as to recognize very small notches for what they are. Thus, to implement a short time constant, a capacitor 4 with the smallest possible capacitance is used. As a result, the discharge current provided by the capacitor 4 is likewise very small. In the far field this time constant is also adequate to reproduce the envelope curve b of the carrier signal XHF very well. A larger discharge current would also have had the disadvantage that the transponder required much more power, which on the whole would lead to a reduction in the range for the data communication.
However, reproducing the envelope curve b of the carrier signal for a transponder operated in the near field is problematic (FIG. 2(b)). In the near field, the amplitude of the envelope curve b is very much greater, with the result that the edges of the envelope curve b are very much sharper in the region of the individual notches a. A much higher discharge current would now be needed to produce the signal voltage c (see dashed characteristic curve) in order to follow the rapid change in the envelope curve. Due to the small capacitor 4 that is present, however, the detector can provide only low discharge currents. This has the immediate result that the voltage dips c are not reproduced at all or are only partially reproduced. This results from the fact that the signal voltage c reproducing the envelope curve b no longer definitely drops to zero because the small capacitor provides only low discharge currents, but these cannot reproduce the rapidly changing carrier signal in the short time required, however. Thus, in the case of a transponder operated in the near field, the voltage dips are no longer recognizable for what they are. This leads to errors in the detection of the corresponding bit information, with the result that a higher bit error rate (BER=Bit Error Rate) must be expected in this constellation.
To avoid this, many RFID systems that are to be operated in the far field of an electromagnetic signal as well as the near field use very high discharge currents for the signal detector regardless of whether the transponder is located in the near field or the far field at the time. However, this has the grave disadvantage that it sharply limits the range of data communication, since a high discharge current is used in the far field as well.
In order to avoid this situation, most existing systems are designed so as to provide the best possible compromise between operation in the far field and operation in the near field. However, this means that the disadvantages such as shorter range, higher bit error rate, and higher energy consumption must also be accepted.
Understandably, there is a desire to avoid this situation.